1. Field of the Invention
The present invention relates to a method for measuring bonding quality of bonded substrates, a metrology apparatus, and a device manufacturing method.
2. Description of the Related Art
In a lithographic manufacturing process, a lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the manufacture of Micro Electronic Mechanical Systems (MEMS), a substrate including mechanical devices, here referred to as top substrate, is bonded on top of a substrate including electronic devices, referred to as bottom substrate. Both the bottom as well as the top substrate may be Silicon On Insulator (SOI) substrates. Currently, SOI substrate bonding technology is used at the manufacture of those substrates. Either qualitative infrared (IR) systems or surface flatness systems are used to assess the substrate bonding quality in order to test for de-lamination. Both of these methods are expensive, and in most cases they are qualitative rather than quantitative.